A quantitative analysis of the negative feedback effects on selected gonadal steroids on gonadotropin secretion is proposed, with emphasis on the relation between serum concentration of steroids and gonadotropins, both in the castrated, steroid treated rat and in the intact animal. This relation will be investigated in both adult and immature rats with the expectation that less circulating steroid will be required for inhibition of gonadotropin secretion in younger animals. Various doses of steroid will be administered via subcutaneous silastic implants containing crystalline testosterone, dihydrotestosterone, or 5a-androstan-3a, 17B-diol (3a-DIOL). Estradiol-17B will be administered via subcutaneous Silastic implants containing a solution of the steroid in peanut oil. Serum LH, FSH and steroid concentrations will be measured by radioimmunoassay and, in the case of the castrated animal, the dose of steroid will be adjusted to maintain serum LH (and FSH in some cases) at precastration levels. In intact steroid treated (male) rats the comparison will be made between the apparent effectiveness of endogenous steroids and exogenous steroids in the inhibition of gonadotropin secretion. In the experiments designed to test the effectiveness of exogenous 5a-reduced androgens as inhibitors of gonadotropin secretion, serum concentrations of both DHT and 3a-DIOL will be measured. It is proposed that such measurements may confirm previous observations from this laboratory which indicate that "non-detectable" serum concentrations of DHT are associated with gonadotropin inhibition in the castrated DHT treated rat and may establish that the rapid conversion of DHT to 3a-DIOL occurs during chronic administration of inhibitory doses of DHT. The DHT:3a-DIOL ratio in the serum of intact and castrated, DHT treated rats will be compared with that measured in other selected species. Other experiments are designed to characterize the development of the pulsitile pattern of LH release following orchidectomy in the rat, the negative feedback effects of pattern by various doses of steroid in the long-term castrate. Attempts will be made to confirm the existence of, and characterize the pattern of, episodic testosterone secretion in the intact male rat with the ultimate goal of comparing the relative effectiveness of "physiological" and non-physiological patterns of serum testosterone in the inhibition of gonadotropin secretion.